Interpretive Summary: Horn flies cause serious economic damage to the U.S. cattle industry. Control of horn flies has relied heavily on chemical pesticides, however, resistance to these pesticides is increasing, suggesting the need for development of new control technology. Previous studies have indicated that the reproductive potential of adult flies is related to their size, with smaller flies being less able to produce large numbers of offspring. Horn fly larvae grow and develop in fresh bovine dung. Although previous efforts have targeted dung as a control site for fly growth using chemicals toxic to the fly larvae, concerns over residues in meat, milk, and the environment have inhibited their widespread acceptance. The present study is the first report relating fly larval growth and developmental success to the nutritional character of the bovine dung. Interestingly, onset of metamorphosis of the fly larvae to adults does not appear to be size-related, sometimes resulting in adult flies that were only ¼ as large as normal. Experimental results suggested that specific substances may increase or decrease larval feeding, meaning that the growth of the fly larvae is subject to external manipulation. Identification of feeding-inhibitors may allow production of tiny, essentially non-reproductive adults. Identification of feeding-stimulants may allow increased larval consumption of specific toxins that they would otherwise find distasteful, thereby increasing the control effectiveness. In either case, identification of these feeding modulator molecules could be the basis for development of new fly control technology.

Technical Abstract:
Aging of bovine dung for 3-5 yr under refrigeration depleted nutrients required for growth and development of horn fly larvae. Growth of horn fly larvae in mixtures of nutrient-depleted dung and fresh dung resulted in production of larvae and pupae stunted in proportion to the relative amount of aged dung. Size reduction of pupae produced in aged dung was due to nutrient limitation, which was partially reversible by supplementation with a bacteriological medium or yeast extract, which increased the numbers of bacteria available for larval consumption. Similar stunting of horn fly larval growth and development was produced by larval crowding in fresh bovine dung. Stunting was dependent on degree of larval crowding, with higher larval densities producing decreased size of individuals, but higher aggregate pupal weight, suggesting that stunting at high larval density resulted from phagoinhibition rather than nutrient depletion of the fresh dung. Successful eclosion of adult horn flies was strongly dependent (0.9578 correlation) on pupal size between 1.0 and 2.7 mg. The lower size limit for pupae which were successful in eclosion of adult flies appeared to be about 1.0 mg, compared to an average weight for horn fly pupae of 4.2 mg. Implications for modeling and control of horn fly populations are discussed.